The muscle spindle has both sensory and motor components.
These motorneurons are classified as static or dynamic according to their pattern of innervation and their physiological effects.
The function of the gamma motor neuron neuromuscular junction is not to supplement the general muscle contraction provided by extrafusal fibers, but to modify the sensitivity of the muscle spindle to stretch. Upon release of acetylcholine by the gamma neuron, the end portions of the intrafusal muscle fibers contract, thus deliberately elongating the non-contractile central portions of intrafusal muscle fibers. This opens stretch-sensitive ion channels of the centrally-positioned sensory axons, leading to an influx of sodium ions. This raises the resting potential of these axons, thereby increasing the probability of action potential firing, thus increasing the sensitivity of the muscle spindle.
When a muscle is stretched, primary sensory fibers (Group Ia afferent neurons) of the muscle spindle respond to both the velocity and the degree of stretch, and send this information to the spinal cord. Likewise, secondary sensory fibers (Group II afferent neurons) detect and send information about the degree of stretch (but not the velocity thereof) to the CNS. This information is transmitted monosynaptically to an alpha efferent motor fiber, which activates extrafusal fibers of the muscle to contract, thereby reducing stretch, and polysynaptically through an interneuron to another alpha motoneuron, which inhibits contraction in the antagonist muscles.
PNF stretching, or proprioceptive neuromuscular facilitation, is a method of flexibility training that reduces the automatic reflex action in order to allow muscles to lengthen.
A muscle spindle model and study the effects of static and dynamic [gamma] stimulations on primary and secondary ending outputs
Jun 01, 2009; ABSTRACT A linear lumped-parameter mechanical model of the muscle spindle is presented. It is shown that the model simulation...
Spinal reflex adaptation in dancers: changes with body orientation and role of pre-synaptic inhibition.(Original Article)(Report)
Oct 01, 2010; Acquisition of dance skills relies on nervous system adaptations in a variety of regions, including the spinal cord. For...